Transmission electron microscopy (TEM) enables observers to form images of extremely small features, on the order of nanometers to fractions of Angstroms. TEM also allows analysis of the internal structure of a sample. In a TEM, a broad beam of electrons impacts the sample, and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site.
A related type of microscopy, scanning transmission electron microscopy (STEM) has similar requirements and capabilities.
A thin TEM sample cut from a bulk sample material is known as a “lamella”. Lamellae are typically less than 100 nanometers (nm) thick, but for some applications a lamella must be considerably thinner. With advanced semiconductor fabrication processes at 30 nm and smaller, a lamella often needs to be less than 20 nm in thickness in order to avoid overlap among small scale structures. Thickness variations in the sample can result in lamella bending, overmilling, or other catastrophic defects. For such thin samples, lamella preparation is a critical step in TEM analysis that significantly determines the quality of structural characterization and analysis of the smallest and most critical structures.
Prior art methods for TEM lamella preparation typically make use of various milling operations performed by a focused ion beam (FIB) system. Such milling operations include cleaning cross-sections, regular cross-sections, and box mills placed in a manner such that the placement of the mill pattern determines final location of an edge of the lamella. The accuracy of lamella thickness and the final lamella center location were based on the accuracy of the placement of these FIB milling operations. In an automated work flow, all milling is typically performed with respect to some feature or fiducial on the top surface of the substrate from which the TEM sample lamella is to be milled.
A known issue involving the production of lamellae in crystalline materials (silicon is a commercially important example) is that a high energy focused ion beam (e.g., 30 kiloelectron volts (keV)) produces a substantial damage layer in the final lamella. The damage layer is caused, for example, by high energy ions discrupting the crystalline lattice of the sample. A known solution is to perform some final processing steps at lower FIB energy, typically 2 keV to 5 keV, but in general not more than 8 keV. These lower FIB energy processing steps are often referred to as “damage removal” steps. In some cases, even lower landing energies (less than 2 keV) are used. In general, the lower the landing energy, the less the disruption of the crystalline lattice and the resulting damage layer thickness decreases with lowered landing energy.
Low landing energy operation is also sometimes referred to as low-kV operation because, if the sample is at ground potential, then the landing energy is directly related to the high voltage potential on the ion source tip.
A problem associated with low-kV (kilovolt) damage removal procedures is that FIB resolution and probe characteristics are substantially degraded at low-kV. The FIB resolution and probe characteristics are degraded because chromatic aberrations typically result in substantial degradations in probe forming performance at low-kV.
This means all steps involving imaging, such as steps used to place the final low-kV damage removal mills, have degraded capability. Typically lamellae are created in automated processes where the placement of low-kV mills critically impacts the final cut placement and thickness precision. The end result is that the control of the placement of edges is much better at 30 kV then it is at low kV, and the process of damage removal introduces undesirably large amounts of uncertainty into thickness and position of the final lamella.